The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
In real life, background noise contaminates pure voice and degrades the performance and capabilities of voice communication systems such as mobile phones, voice recognition, voice coding, speaker recognition and the like. Accordingly, research on sound quality improvement to reduce noise effects and enhance system capabilities has progressed over time, and the importance thereof currently receives a lot of attention.
Meanwhile, a Spectral Subtraction (SS) is a typical method widely used in a single channel due to its low cost and easy implementation among various sound quality improving methods. The inventors have noted that in the spectral subtraction there might remain musical noise corresponding to a new artifact sound in the voice signals after the spectral subtraction.
The musical noise refers to a random frequency component generated by evaluating estimated noise as being lower than original noise, and furthermore refers to a tone which perceivedly annoys a listener since residue of the musical noise on time and frequency axes in a spectrogram is discontinuously spread.
In this connection, in order to suppress the residue of the musical noise, the spectral subtraction method based on a gain function has been proposed. The inventors have noted that most of the proposed methods might not be able to efficiently improve sound quality in a noise environment of a low Signal to Noise Ratio (SNR). It is probably because the improved voice still has musical noise and/or low speech intelligibility.
Accordingly, success and failure of the sound quality improvement using a gain function based-Spectral Subtraction (SS) may be determined according to an accurate gain function setting by which a small loss of voice signal is generated and the residue of musical noise is suppressed.
Meanwhile, in a voice communication system, a sending frequency response (SFR) filter has a function to enhance or weaken a response of a particular frequency band in order to reproduce corresponding voice as much as possible by providing a flat frequency response pattern for the provided voice signal. The inventors have noted that when the voice improved through the gain function based-spectral subtraction (SS) method is filtered by the SFR filter function, in an enhanced band, not only the voice but also the noise are enhanced and thus large noise might be heard by a listener, and, inversely, in a weak band, not only the noise but also the voice is weakened and thus the listener might experience low speech intelligibility.